CS1001
Lecture 14
1
Overview


Java Programming
Arrays
2
Goals


Understand the basics of Java
programming
Control Statements and Arrays
3
Assignments


Brookshear: Ch 4, Ch 5 (Read)
Read linked documents on these slides
(slides will be posted in courseworks)
4
Arithmetic





Operators: +, -, /, * , %
The precedence of operators and
parentheses work the same way as in
algebra.
m % n means the remainder when m is
divided by n (e.g. 17 % 5 is 2).
% has the same rank as / and *
Same-rank binary operators are
performed in order from left to right.
5
Arithmetic (cont’d)


The type of the result is determined by the
types of the operands, not their values; this
rule applies to all intermediate results in
expressions.
If one operand is an int and another is a
double, the result is a double; if both
operands are ints, the result is an int.
6
Arithmetic (cont’d)

Caution: if a and b are ints, then a / b is
truncated to an int…
17 / 5 gives 3
3 / 4 gives 0

…even if you assign the result to a double:
double ratio = 2 / 3;
The double type of
the result doesn’t
help: ratio still gets
the value 0.0.
7
Arithmetic (cont’d)

To get the correct double result, use
double constants or the cast operator:
double ratio = 2.0 / 3;
double ratio = 2 / 3.0;
double factor = (double) m / (double)
n;
double factor = m / (double) n;
Casts
double r2 = k / 2.0;
double r2 = (double) k / 2;
8
Arithmetic (cont’d)


Caution: the range for ints is from
-231 to 231-1 (about -2·109 to 2·109)
Overflow is not detected by the Java
compiler or interpreter
n
n
n
n
n
n
n
=
=
=
=
=
=
=
8
9
10
11
12
13
14
10^n
10^n
10^n
10^n
10^n
10^n
10^n
=
=
=
=
=
=
=
100000000
1000000000
1410065408
1215752192
-727379968
1316134912
276447232
n!
n!
n!
n!
n!
n!
n!
=
40320
=
362880
=
3628800
=
39916800
= 479001600
= 1932053504
= 1278945280
9
Arithmetic (cont’d)

a
a
a
a
a
Use compound
assignment
operators:
=
=
=
=
=
a
a
a
a
a
+ b;
- b;
* b;
/ b;
% b;
a
a
a
a
a
+= b;
-= b;
*= b;
/= b;
%= b;

Use increment
and decrement
operators:
a = a + 1;
a = a - 1;
a++;
a--;
Do not use these in
larger expressions
10
Review:


What is a variable?
What is the type of variable that holds an
object?
11
Review (cont’d):





What is the range for ints?
When is a cast to double used?
Given
double dF = 68.0;
double dC = 5 / 9 * (dF - 32);
what is the value of dC?
When is a cast to int used?
Should compound assignment operators
be avoided?
12
Objectives:



Learn about arrays and when to use
them
Learn the syntax for declaring and
initializing arrays
Learn how to access array’s size and
elements
13
What is an Array



An array is a block of consecutive memory
locations of the same data type.
Individual locations are called array’s
elements.
Sometimes when we say “array’s element”
we mean the value stored in that element.
1.39
1.69
1.74
0.0
An array of
doubles
14
What is an Array (cont’d)


Rather than treating each element as a
separate named variable, the whole array
gets one name.
Specific array elements are referred to by
using array’s name and the element’s
number, called index or subscript.
1.39
1.69
1.74
0.0
c[0]
c[1]
c[2]
c[3]
o is array’s
name
15
Indices (Subscripts)



In Java, an index is written within square
brackets following array’s name (e.g., a[k]).
Indices start from 0; the first element of an
array a is referred to as a[0] and the n-th
element as a[n-1].
An index can have any int value from 0 to
array’s length - 1.
16
Indices (cont’d)

We can use an int variable or any
expression that evaluates to an int value
as an index:
a [3]
a [k]
a [k - 2]
a [ (int) (6 * Math.random()) ]
17
Indices (cont’d)


In Java, an array is declared with fixed
length that cannot be changed.
Java interpreter checks the values of
indices at run time and throws
IndexOutOfBoundsException if an index is
negative or if it is greater than the length
of the array - 1.
18
Why Do We Need Arrays?

1000
times!
The power of arrays comes from the fact that
the value of a subscript can be computed
and updated at run time.
Before (no arrays):
After (with arrays):
int sum = 0;
sum += score0;
sum += score1;
…
sum += score999;
int n = 1000;
int sum = 0, k;
for (k = 0; k < n; k++)
sum += scores[k];
19
Why Arrays? (cont’d)

Arrays give direct access to any element —
no need to scan the array.
Before (no arrays):
After (with arrays):
if (k == 0)
display (score0);
else if (k == 1)
display (score1);
else
… // etc.
display (scores[k]);
20
Arrays as Objects


In Java, an array is an object. If the type
of its elements is anyType, the type of the
array object is anyType[ ].
There are two ways to declare an array:
anyType [ ] arrName;
or
The difference becomes significant
only when several variables are
declared in one statement:
int [ ] a, b; // both a, b are arrays
anyType arrName [ ];
int a [ ], b; // a is an array, b is not
21
Arrays as Objects (cont’d)


As with other objects, the declaration creates
only a reference, initially set to null. An array
must be created before it can be used.
There are two ways to create an array:
arrName = new anyType [ length] ;
Brackets,
not parens!
or
arrName = new anyType [ ] { val1, val2, …, valN };
22
Declaration and Initialization

When an array is created, space is allocated
to hold its elements. If a list of values is not
given, the elements get the default values.
scores = new int [10] ;
// length 10, all values set to 0
words = new String [10000];
// length 10000, all values set to null
23
Initialization (cont’d)

An array can be declared an initialized in one
statement:
int scores [ ] = new int [10] ;
// length 10
private double gasPrices [ ] = { 1.49, 1.69, 1.74 };
String words [ ] = new String [10000];
String cities [ ] = {"Atlanta", "Boston", "Cincinnati" };
24
Initialization (cont’d)

Otherwise, initialization can be postponed
until later:
String words [ ];
// not yet initialized
...
words = new String [ console.readInt() ];
private double gasPrices [ ]; // not yet initialized
…
gasPrices = new double [ ] { 1.52, 1.69, 1.75 };
25
Array’s Length




The length of an array is determined when
that array is created.
The length is either given explicitly or comes
from the length of the {…} initialization list.
The length of an array arrName is referred to
in the code as arrName.length.
length appears like a public field (not a
method) in an array object.
26
Review:





Why are arrays useful?
What types of elements can an array have?
How do we refer to an array’s element in
Java?
What happens if an index has an invalid
value?
How do we refer to the length of an array?
27
Sorting
28
Alternate Sorting Algorithm
(Selection Sort)
main(String[] args) {
int inputSize = args.length;
int[] sortedArray = new int[inputSize];
for (i=0; i < inputSize; i++) {
(1) Loop through args, find largest element and
remember its index (ie for (int j = 0; …))
(2) Put the largest element into the ith location of
sortedArray;
(3) Delete (set to -1) the ith element of args
}
http://www.ee.unb.ca/brp/lib/java/selectionsort/
29